As image forming apparatuses such as printers, facsimile machines, copying machines, plotters, and multifunction peripherals including functions of these devices, for example, there are image forming apparatuses of a liquid ejection recording type, having a print head to eject ink droplets. As such image forming apparatuses, inkjet recording apparatuses and the like are widely known. In the image forming apparatuses of the liquid ejection recording type, ink droplets are ejected from the print head onto carried paper (OHP transparencies and the like are included, to which ink droplets, other liquid, and the like can adhere. These are also called a medium to be recorded, a recording medium, recording paper, a recording sheet, and the like) to form images (used to refer to recording, printing text, imaging, and printing). In serial image forming apparatuses of the liquid ejection recording type, images are formed by ejecting droplets from a print head moving in a horizontal direction. In line type image forming apparatuses of the liquid ejection recording type using a line head, droplets are ejected by a fixed line print head.
In the invention, the “image forming apparatus” means an apparatus to form images by ejecting liquid to a medium such as paper, a thread, a fiber, fabric, leather, metal, plastic, glass, wood, and ceramics. “Forming images” means not only to provide a medium with an image with a meaning such as text and figures, but also an image without a meaning such as a pattern (droplets are simply ejected onto a medium). Moreover, “ink” is not limited to regular known inks, but is a general term for all liquid which can be used for forming images.
A general structure of such an image forming apparatus of the liquid ejection type includes a subtank (also called a head tank or a buffer tank) serving as a small liquid container to supply ink to a print head provided on a carriage. A main tank with a large capacity (also called a main cartridge or an ink cartridge) is provided in a main body of the image forming apparatus to supply (charge) ink to the subtank.
For example, Patent Document 1 discloses an image forming apparatus having a subtank type ink supplier including an internal spring and at least one wall formed of a film material. In this apparatus, an open valve of the subtank is opened when ink is supplied to the subtank and the open valve is closed when a negative pressure is generated to eject the ink.
[Patent Document 1] Japanese Patent Application Publication No. 2005-059274
Patent Document 2 discloses an image forming apparatus having a displaceable negative pressure sensor lever which displaces in accordance with the negative pressure in a subtank, and a method to control the ink supply to the subtank by sensing the displacement of the negative pressure sensor lever by an optical sensor. If the optical sensor senses the negative pressure sensor lever, ink is sent from a main tank to the subtank.
[Patent Document 2] Japanese Patent Application Publication No. 2007-015153
Patent Document 3 discloses a technique to control driving and stopping of a driving motor by a supply pump driver circuit in response to a sensor signal from a sensor. The driving motor is stopped in response to a sensor signal of a load sensor which senses a load of the supply pump as a load of the driving motor.
[Patent Document 3] Japanese Patent Application Publication No. 2007-105935
Patent Document 4 discloses an image forming device having a valve unit which opens and closes an ink supply path from an ink tank to a print head. A valve controller determines time to open the valve so that ink is supplied at an amount corresponding to an ink amount ejected by the print head.
[Patent Document 4] Japanese Patent Application Publication No. 2007-050565
Patent Document 5 discloses an image forming apparatus having a pump to supply ink, a motor to drive the pump, and an input current supplier to supply the motor with an input current. A current value of the input current is changed in accordance with a position of a moving part of the pump in its movable area so that an operation rate of the moving part of the pump becomes constant in each cycle of the pump.
[Patent Document 5] Japanese Patent Application Publication No. 2006-264239
Patent Document 6 discloses a technique to determine the existence of ink in a subtank when the ink surface is sensed a predetermined number of times and determine the absence of ink in the subtank when the ink surface is not sensed the predetermined number of times.
[Patent Document 6] Japanese Patent Application Publication No. 2006-123365
In Patent Document 2, the negative pressure sensor lever which displaces in accordance with the ink supply is provided. When the optical sensor set at a desired level of the ink supply senses the negative pressure sensor lever, the supply motor to drive the supply pump is stopped to stop the ink supply. In this case, however, there is a delay in response of about several msec to several 100 msec after an instruction to stop the supply motor is made until the supply motor actually stops driving. Therefore, ink more than the desired level is supplied to the subtank during the response delay.
A detailed description is made with reference to FIG. 27. FIG. 27a shows a voltage applied to the driving motor to drive the supply pump and FIG. 27b shows a revolution of the driving motor. FIG. 27c shows a sensor signal outputted by the optical sensor as an amount sensor by sensing the negative pressure sensor lever which displaces in accordance with an ink amount in the subtank (the signal is outputted when the ink is at a predetermined amount). FIG. 27d shows an actual operation rate of the supply pump. As shown in FIG. 27, a driving voltage Vin1 is applied to drive the driving motor at a revolution of N1 and operate the supply pump at an operation rate of Vp1, thereby ink is supplied to the subtank. When the voltage supply to the driving motor is stopped by receiving the sensor signal inputted by the amount sensor, indicating that the ink is at a predetermined level, time is required until the supply pump actually stops operation. This time varies in the range from time Tts to Ttd. Ink continues to be supplied until the supply pump stops. FIG. 28 shows an example of using an amount sensor to sense an ink surface in the subtank, by which similar behavior is expected.
FIG. 29 shows a relationship among the voltage (driving voltage) applied to the driving motor which drives the supply pump, an ink supply rate (supply rate: a rate to supply ink to the subtank), and time required for the supply pump to stop. When the voltage (driving voltage) applied to the driving motor is raised to increase the ink supply rate, more time is required for the supply pump to stop. When the driving voltage applied to the driving motor is decreased to shorten the time required for the supply pump to stop, a desired ink supply rate (minimum amount of the ink supply) cannot be obtained (it takes time to supply the ink).
FIG. 30 shows the case of applying a relatively high driving voltage to the driving motor (here, a duty ratio is increased). In this case, time tdl is required for a motor driving rate to be zero after the driving voltage is set zero. In FIG. 31, the driving voltage applied to the driving motor is set relatively low (here, the duty ratio is decreased). In this case, time tds is required for the motor driving rate to be zero after the driving voltage is set zero. The time tdl is longer than the time tds (time required for the driving motor to stop varies too).
In this manner, ink more than the desired level is supplied to the subtank (too much ink supply) until the supply pump stops. The amount of the excessive ink supply varies depending on the variations of time required for the supply pump to stop. Moreover, when the supply pump is a piston pump, ink is supplied in a pulsated manner. Therefore, the final amount of ink supply varies largely from the desired level.
In this case, the variations in the amount of ink supply can be decreased by slowing down the ink supply rate as described above. With the slow ink supply rate, however, it takes longer to supply the ink, which also affects a recording rate. When the amount of ink supply largely varies, the ink may flow out of the subtank from the open valve. When the ink supply is stopped earlier to avoid the ink from flowing out of the subtank, the ink is not supplied to the full desired amount. As a result, the capacity of the subtank is not efficiently used.